The Life of a Countertop Croc

Why a raven is like a writing desk – the infamous riddle put forward by Lewis Carroll’s Mad Hatter – was meant to be an incomprehensible mystery, but a fortuitous discovery made half a century ago answers the un-asked question of how a crocodile is like a countertop.

Nodular limestone is beautiful. The sedimentary looks like marble when polished, and slabs of this stone quarried from Portomaggiore in Ferrara, Italy have been used in sidewalks, countertops, and other bits of architecture. What few realize, however, is that the limestone from this particular place are the remnants of a Jurassic sea – a fact the quarry owner was reminded of when he spotted a sliced-up skull among the rocks in 1955.

The skull and a few associated bones had been accidentally divvied up into four cross-sections, as if the slabs were archaic versions of the CT scans paleontologists now use to look inside fossil bones. What the sliced pieces belonged to was not immediately clear, but in 1956 the paleontologist Piero Leonardi figured out that the bones belonged to a crocodile unlike any of its modern-day relatives. Part of an entirely-extinct group called metriorhynchids, the “Coccodrillo di Portomaggiore” was highly-modified for life at sea.

The skeleton of the marine croc Cricosaurus (top) compared with that of the ichthyosaur Stenopterygius (bottom). Notice that both have tailbends to support a large tailfin. From Young et al, 2010.

Among modern species, at least, a croc is a croc is a croc. Living crocodylians – the crocodiles, alligators, caimans, and gharials of the world – generally have the same squat body plan, with the major differences between them being in the shape of their low-profile skulls. There exists a diversity of modern crocodylians, but the disparity between them is relatively low. This has not always been the case.

Crocodylians in their modern form have been around for about 85 million years, but, since the earliest members of the croc line split off from other archosaurs over 245 million years ago, the wider croc family displayed a startling variety of forms. There were 225 million year old bipedal crocs that superficially looked like dinosaurs, a 105 million year old species with cat-like teeth fitted in its short skull, Cretaceous forms with armadillo-like armor plating, and many more, including the metriorhynchids.

Fossil specimens of marine-adapted crocs that sculled the seas from about 175 million years ago through 135 million years ago are somewhat similar to the anatomy of their distant, modern cousins, but the metriorhynchids had some peculiar specializations that made them different. They lost most of the bony armor plating – osteoderms – which covers the bodies of modern crocodiles, and their ends of their tails were kinked downwards to support a large tail fin to propel them through the water. Their limbs also became more fin-like, and, as Robert Gandola and colleagues reported in 2006, metriorhynchid crocs had large salt-excretion glands on their heads to cope with life at sea.

One of the slabs containing skull and jaw fragments of Neptunidraco (left) with a line drawing of the parts (right). From Cau and Fanti, 2011.

At about 170 million years old, the Coccodrillo di Portomaggiore was one of the earliest seafaring crocs known. Contrary to earlier reports, though, this sectioned specimen did not belong to an already-known genus. The fossil had previously been identified as Metriorhynchus or Geosaurus – two well-known varieties of marine croc – but a reexamination of the specimen by paleontologists Andrea Cau and Federico Fanti a few months ago found it to be an entirely new species. They named the animal Neptunidraco ammoniticus.

Though the skull was sectioned into four parts, the head of Neptunidraco was complete enough for Cau and Fanti to determine that this animal was intermediate in form between slender-snouted forms like Metriorhynchus and exceptionally deep-skulled species such as the fearsome Dakosaurus. Even so, the streamlined skull was Neptunidraco was that of a predator that quickly snapped up quick-moving prey rather than crushed bone or slashed off flesh – a hypothesis supported by tooth fragments suggestive of thick, pointed, and slightly recurved dental armament.

Despite the disparity between the skulls of Neptunidraco and large-prey specialists like Dakosaurus, though, the two were relatively close cousins. Both belonged to a subgroup of the marine crocs called geosaurines, which contrasted with a second subgroup called the metriorhynchines, and species in each subgroup exhibited different specializations for disparate types of prey. As recently reported by paleontologists Mark Young, Mark Bell, and Stephen Brusatte, the relatively gracile-skulled species in the metriorhynchines were best-suited to feeding on fish and squid, whereas the geosaurines typically had more robust skulls adapted to chomping down on larger prey. This is not to say that all geosaurines were doing the same thing. Whereas Dakosaurus had a crushing bite, Geosaurus was a predator that slashed flesh off prey. In general, though, the geosaurines were after larger menu items than more slender-snouted forms of marine croc.

The differences in diet between the robust and slender-snouted crocodiles would seem to make Neptunidraco an outlier. Though more closely related to Geosaurus and Dakosaurus, the tooth fragments Cau and Fanti described may indicate a diet like that of a narrower-skulled form. (It should be noted, though, that complete teeth were not preserved, and so the exact details of the animal’s teeth have yet to be fully elucidated.) Since Neptunidraco was one of the earliest members of its subgroup, though, this indicates that the anatomical differences in Geosaurus and Dakosaurus were specializations that were derived from a fish-eating ancestral state. Viewed this way, through the lens of evolution through Deep Time, the anatomy of Neptunidraco might help elucidate how the baddest of all the oceangoing crocs got their start.

Top Image: A restoration of the metriorhynchid croc Suchodus by Dmitry Bogdanov. Image from Wikipedia.